The present invention relates to a developing means to develop a latent image on an image forming member, especially, the similar member employed in electrophotography, and more specifically relates to an improved method of forming a developer layer whereby the thickness of developer being carried and transported on a developer transporting member is regulated.
In the electrophotography in which an electrostatic latent image on an image forming member is visualized through development to obtain final image, the current developing methods are roughly categorized into two developing methods, one of which is a method of using one-component developer and the other is a method of using two-component developer comprising magnetic carrier and non-magnetic toner.
Among the developing methods to use one-component developer, a developing method using one-component non-magnetic developer has difficulty in forming a stable and uniform developer layer, because the force to carry the developer on a developer transporting member solely comprises Coulomb force and van der Waals force derived from electrical charge the developer has. Additionally, with a developing method using one-component magnetic developer, it is difficult to obtain vividly colored toners other than black one, because the toner generally contains black colored magnetite powder as magnetic material. On the other hand, the developing method to use two-component developer has, when compared with other methods, an advantage capable of forming a stable developer layer and using vivid color toners.
Generally speaking, when developing a latent image on an image forming member with a developer carried and transported on a developer transporting member, the smaller distance between a latent image carrying member and a developer transporting member can sharpen contrast of an image obtained, realizing the improved image quality. In a contact-developing method wherein the development is carried out by making the image carrying member in contact with the developer layer, when the distance between the image carrying member and the developer layer is set smaller, it is necessary to limit the amount of developer being carried and transported on the developer transporting member within a proper range in order to prevent developer from being pressed to form a solid between the image carrying member and the developer transporting member or to protect the latent image or the developed image against damage caused by such a pressure. Incidentally, the term "amount of developer" refers to the weight per unit area of developer being carried on the surface of developer transporting member. Additionally, even in a non-contact-developing method wherein the development is carried out by keeping the image carrying member detached from the developer layer, if the distance between the image carrying member and the developer layer is set smaller, it is necessary to limit the amount of developer on the developer transporting member within a proper range so that the image carrying member may not contact the developer layer.
In conventional developing methods using two-component developer, the development have been carried out with, for example, a developing unit having the following constitution. A cylinder-shaped nonmagnetic member or nonmagnetic belt is used as a developer transporting member, and magnets are disposed on the opposite side of the developer carrying surface of the developer transporting member. Such magnets may be, by an arrangement whether they are disposed fixedly or rotatably in relation to a certain axis, arranged to satisfy the conditions of development. 0f developer being carried on the developer carrying surface, the magnetic carrier receives force from the field of magnets disposed opposite to the developer carrying surface and rises to form the fur in accordance with the force of magnets as well as the magnetism of carrier, thus, the carrier is retained on the developer carrying surface. Being agitated in a developer container, magnetic toner is rubbed with magnetic carrier and is given triboelectricity. Accordingly, nonmagnetic toner and magnetic carrier are attracted with each other due to Coulomb force as each of them is provided with an electrical charge of which polarity is opposite to each other. As magnetic carrier is retained on the developer carrying surface due to the field of magnets, nonmagnetic toner together with magnetic carrier is retained on the developer carrying surface, and, accordingly, the developer layer comprising magnetic carrier and nonmagnetic toner is formed on the developer carrying surface. The developer layer is retained and transported on the developer carrying surface to develop the latent image on the image carrying member.
Such a developer layer should, as mentioned above, satisfy the minimum requirements that the layer is not so pressed as to form a solid through the gap between the image carrying member and the developer transporting member, and that the image carrying member and the developer layer both of which should be kept separate with each other are kept in no contact.
Furthermore, the developer layer should satisfy the following requirements:
(1) "Fogging" phenomenon where toner deposits on the surface of image carrying member other than the image area does not occur. PA0 (2) "Carrier adhesion" phenomenon where carrier deposits on the surface of image carrying member does not occur. PA0 (3) Proper amount of toner to be able to provide an image having enough density deposits on the latent image area. PA0 (4) The deposited toner image has gradient sufficient to reproduce the gradient of a latent image.
To satisfy these requirements, the properties of developer, the agitating method of developer, the properties of latent image carrying member, the relative velocity between the latent image carrying member and developer layer, the means for forming a latent image on the latent image carrying member, electrical bias applied across the image carrying member and the developer transporting member, the force of magnets, the configuration of magnetic field, and the like should be kept within adequate ranges. Such requirements further include the amount of developer on developer carrying member and the thickness of developer, whose adequate ranges are much smaller than those for the minimum requirements.
A conventional layer thickness regulating means was so constituted that, in order to provide the required amount and thickness of developer layer retained on the outer circumference surface of a developer carrying member, a developer layer thickness regulating plate 205 made of metal or resin and shown in FIG. 20 was disposed in a close proximity to the surface of developer transporting member 203, and that developer D is carried while being retained on the surface of developer transporting member 203 and is transported to the developing area after passing trough the gap between the layer thickness regulating plate 205 and the surface of developer transporting member 203. . The developer transporting member 203 shown in the figure is one example wherein the member comprises a nonmagnetic cylinder internally housing magnets 204.
To regulate the layer thickness of developer D at approx. 1 mm or less by using such a layer thickness regulating plate 205, the gap distance between the layer thickness regulating plate 205 and the surface of developer transporting member 203 should be set at approx. 0.5 mm or less. Because, developer D passes through the gap between the developer layer thickness regulating plate 205 and the developer transporting member 203 in the form of compacted higher density developer layer, but, contrary, developer D on the surface of the developer transporting member 203 after passing through the gap between the layer thickness regulating plate 205 and the developer transporting member 203 rises due to the force of magnets 204 to form a lower density developer layer with thickness 1.5-3 times as large as that of the layer passing through the gap between the thickness regulating plate 205 and the developer transporting member 203.
Ordinary photocopying machines, printers and the like which develop and visualize an electrostatic latent image have a several hundred mm of image width. To set the gap, mentioned previously, between the layer thickness regulating plate and the surface of developer transporting member at approx. 0.5 mm or less and to maintain it evenly and stably across all the span of image width necessitates high-precision manufacture and adjustment of the layer thickness regulating plate, which cannot be easily carried out.
Therefore, it is the object of the present invention to provide a method for forming developer layer, wherein a high-quality image is obtainable by using developer which may use vivid color toner and comprises magnetic carrier and nonmagnetic toner being capable of forming a stable developer layer, and by evenly and stably forming a thin developer layer which can allow to set a latent image carrying member unconventionally closer to a developer transporting member.